1. Field of the Invention
The invention relates to a method of fabricating a semiconductor device, and more particularly to such a method including a step of filling a contact hole and/or a through-hole formed throughout an interlayer insulating film with TiN and/or Ti films formed by chemical vapor deposition.
2. Description of the Related Art
With integration in LSI becoming higher and higher, a contact hole has been formed smaller and smaller in a diameter, resulting in that an aspect ratio defined by a depth divided by a diameter of a contact hole becomes greater and greater. Thus, since metal such as aluminum formed by sputtering, which has been conventionally employed, has poor step coverage, a connection resistance has become greater, and disconnection between upper and lower wiring layers has often happened. Even if upper and lower wiring layers could be electrically connected with each other through a contact hole filled with aluminum, there is a problem of poor reliability that disconnection often happens between the wiring layers because of electromigration, namely a phenomenon where aluminum atoms are moved by a current running through aluminum.
As a solution to this problem, a contact hole has been filled with metal. A typical solution is a so-called tungsten plug method by which a contact hole is filled with tungsten formed by chemical vapor deposition (hereinafter, referred to simply as "CVD") which has excellent step coverage. The tungsten plug method includes the steps of forming a barrier metal film by sputtering, filling a contact hole with tungsten by CVD, and etching back tungsten to allow tungsten to remain only in the contact hole to thereby form a tungsten plug. Herein, the barrier metal is composed of titanium (Ti) for lowering a connection resistance of a contact hole or a contact resistance, and a titanium nitride (TiN) for increasing adhesion between a contact hole and tungsten to thereby prevent tungsten from penetrating a substrate.
However, as a contact hole has been formed smaller and smaller in a diameter with an aspect ratio becoming higher and higher, it has become impossible to form a titanium or titanium nitride film to have a desired thickness in a contact hole even in accordance with sputtering in the above-mentioned tungsten plug method, resulting in problems that a contact resistance is increased, and that a device may be destroyed by tungsten.
Hence, many attempts have been made to form by CVD, a titanium or titanium nitride film having excellent step coverage. However, three layers, namely a titanium layer, a titanium nitride layer, and a tungsten layer have to be formed by CVD in such attempts, resulting in problems that fabricating steps become complicated, and that fabrication costs are unavoidable increased.
As a solution to these problems, there has been suggested a method in which a contact hole is filled with titanium nitride or titanium by CVD having excellent step coverage, to thereby omit a step of forming a tungsten film.
FIGS. 1A to 1D illustrates the suggested method including the step of filling a contact hole with titanium nitride. First, an interlayer insulating film 82 is formed by CVD on a silicon substrate 81 on which devices are fabricated. Herein, the interlayer insulating film 82 is made of a boron phospho silicate glass (BPSG) film comprising a silicon dioxide film into which phosphorus (P) and boron (B) are added. Then, as illustrated in FIG. 1A, a contact hole 80 is formed by conventional photolithography and dry etching so that the contact hole 80 reaches the silicon substrate 81. The contact hole 80 is designed to have a diameter of about 0.4 .mu.m.
Then, a titanium (Ti) film 83 is deposited over the BPSG film 82 by plasma-enhanced CVD by a thickness in the range of 10 nm to 50 nm. Subsequently, a titanium nitride (TiN) film 84 is deposited over the Ti film 83 by thermal CVD by a thickness of about 0.3 .mu.m. Thus, as illustrated in FIG. 1B, the contact hole 80 is completely filled with the Ti film 83 and the TiN film 84.
Then, the Ti film 83 and the TiN film 84 situated above the BPSG film 82 are removed by dry etching using chlorine (Cl.sub.2) gas. Thus, the Ti film 83 and the TiN film 84 remains only in the contact hole 80, as illustrated in FIG. 1C.
Then, an aluminum alloy film 85 is deposited over the BPSG film 82 by sputtering, and then patterned in a desired pattern by conventional photolithography and dry etching. Thus, as illustrated in FIG. 1D, an aluminum wiring layer is formed.
The method of filling a contact hole with a TiN film formed by CVD has been suggested, for instance, in Japanese Unexamined Patent Publications Nos. 5-94965, 5-94969 and 5-136085.
Though the above-mentioned method has suggested only a method of filling a contact hole with a TiN film, it is considered that a contact hole could be filled with a Ti film formed by CVD in accordance with almost the same method as the above-mentioned one.
In accordance with the above-mentioned conventional method of fabricating a semiconductor method, if a TiN film was formed thick to fill a contact hole therewith by CVD, there would arise a problem that the TiN film may be cracked or peeled off, because a tensile stress equal to or greater than 10.sup.10 dyne/cm.sup.2 acts on the TiN film having been formed by CVD, and the TiN film has poor adhesion with a silicon dioxide film. If the TiN film was peeled off, the underlying interlayer insulating film or BPSG film was improperly etched in a subsequent step of etching the TiN film, resulting in reduction in fabrication yield and reduction in reliability. In addition, the peeled-off TiN film acts as a debris, which also causes reduction in fabrication yield. When the TiN film was cracked, there would also arise a problem that the underlying BPSG film would be improperly etched.
When a contact hole is to be filled with a Ti film formed by CVD, there would arise the same problems as those for a TiN film. Namely, a Ti film can be peeled off or cracked.
It is quite important for ULSI such as DRAM to have a greater cell capacity as well as to have a higher aspect ratio for a contact hole. In order to ensure a higher capacity, a method of reducing a thickness of an oxide film in equivalence of a thickness of a capacitive insulating film is now being developed. Among oxide films, a tantalum oxide (Ta.sub.2 O.sub.5) film is considered promising as material a film made of which is capable of reducing a thickness thereof in comparison with a SiN film.
However, in this method, after a ultra-thin Ta.sub.2 O.sub.5 film has been once formed, it is not allowed to apply a heat treatment at 500.degree. C. or higher to a semiconductor device in order to protect a current leakage characteristic. Hence, a Ti or TiN film for filling a contact hole or a through-hole therewith is required to be formed by CVD at substrate temperature equal to or smaller than 500.degree. C. However, it would be quite difficult or almost impossible to have sufficient adhesion between a Ti or TiN film and a through- or contact hole at such low substrate temperature, which would make more serious a problem that a Ti or TiN film is cracked or peeled off on an interlayer insulating or BPSG film. A Ti or TiN film is cracked or peeled off often particularly in a wide area where no patterns are formed.
Another method of forming a Ti or TiN film by CVD has been suggested in "Integrated CVE titanium and titanium nitride processes for sub-0.5- .mu.m metallization" by Joseph Hillman et al., Solid State Technology, July 1995, pp. 147-152. The suggested CVD-Ti and CVD-TiN processes with an existing CVD-tungsten (W) process provides a high-quality metallization module for devices with high-aspect-ratio features at a low cost relative to collimated sputtering.